1. Field of the Invention
The present invention relates to systems for reproducing sound, and more particularly relates to a system for the reconstruction of pyscho-acoustic images in stereophonic sound reproduction systems.
2. General Background and Prior Art
Numerous prior publications and patents have disclosed systems and devices which have the objective of distortion free reproduction of recorded sonic materials typically in the home environment. These systems usually introduce some distortion.
One type of distortion is imaging distortion, typically manifested as a "flattening" of the sound stage image, that is, a tendency to hear as if they emanate from a single plane which when recorded were in widely scattered locations throughout the recording area, or stage sound sources. Even with the finest stereo system, one can sense only general depth, that is a vague "closer" or "farther" perception, regardless of the dispersion of the original sound sources. Because stereo systems use only two sound sources, the resultant imaging is necessarily different from randomly placed multiple sound sources.
One proposed, but impractical, "ideal" playback system uses one speaker for reproducing the sound of each source, each speaker being placed in the exact location originally occupied by the respective source with respect to the listener.
A more practical but less satisfactory approach is the well known quadraphonic system, in which separate information channels drive each of four speakers placed as to generally surround the listener. The conventional stereophonic system is simpler yet, but represents an even less satisfactory solution to the sound imaging problem.
These systems fail to accurately reproduce the sound image because they fail to take into consideration the manner in which the ears discern location in space. All sounds which strike the ear drums, or timpanic membrane, have first been partially supplemented by the wing of the outer ear, or auricle. The auricle functions as a reflector or funnel which modifies a portion of the sound entering the ear canal, or external auditory meatus, according to the direction and distance of the sound from the listener.
For a given sound originating in front of the listener, a certain portion of the sound ("direct-in" or D.I. signal) will enter the ear canal opening directly. Due to losses by multiple reflections as the sound travels through the ear canal, higher and lower frequencies will be attenuated relative to the mid-range frequencies according to the angle at which the sound subtends the opening. This attenuation is in addition to the characteristic attenuation of these frequencies due to the distance from the listener.
The other, usually much smaller portion of the sound reaching the timpanic membrane ("apparent-on-axis" or A.O.A., signal) will be reflected off the auricle, more or less directly into the ear canal by that section of the auricle directly adjacent to the ear canal opening and above the ear lobe. Because this sound component undergoes essentially a single reflection, it does not suffer the frequency abberations of the D.I. signal.
The subjective ratio of A.O.A. signal to D.I. signal is dependent upon both distance and angle of the sound source relative to the facing direction of the listener. The subjective level of A.O.A. signal increases as distance is reduced because of (1) an absolute level increase; (2) the high frequency levels increase relative to the mid-range level and, (3) to a lesser degree, low frequency levels increase relative to the mid-range level. High frequency information is lost through friction as it passes through air, and low frequency information is lost with distance because of the omnidirectionability of low frequency energy in air. Since level changes are more apparent to a listener at high frequencies than at lower frequencies, the listener thus hears both a subjectively and absolutely increasing A.O.A. component as the distance to the sound source is reduced.
The A.O.A. component also increases relative to the D.I. signal as the sound source moves closer to being in front of the listener. The D.I. component enters the meatus less directly and is therefore attenuated more before reaching the eardrum than is the reflected A.O.A. components.
As the A.O.A./D.I. ratio changes, the brain discriminates between angle and distance changes on the basis of the sound information of both ears. Simultaneous similar A.O.A. changes indicate distance change, while simultaneous dissimilar A.O.A. changes indicate change in the angular location of the source.
In addition, as distance is reduced, a subjective effect taught by the well known Fletcher-Munson curves comes into play. The relative levels of the higher and lower frequencies increase relative to mid-range because of increasing sound levels. It is hypothezied that this effect is intimately connected with the brain's ability to perceive the distance of a sound.
Conventional stereophonic systems do not take into account this sophisticated localizing function of the human ears. Although some localizing information is provided by level differences between two speakers, the illusion of location is imperfect and at best the sound stage image is flat. In fact, the listener tends to hear each speaker as a separate sound source. The quadraphonic system similarly does not address itself to this problem, and merely adds two more point sources.
While ideally all recordings should be made with the equivalent of two microphones close together so as to record the sound information which would be heard by two ears, many recordings are made with distantly spaced microphones and/or improper mixing techniques, and thus have lower than ideal interchannel crosstalk. Reproduced through conventional stereophonic systems these recordings have no discernable image.
When such non-ideal recordings are played through stereophonic speakers which have been "toed-in" (angled inward) image loss is less severe. However toe-in of front speakers creates overall incorrect image location (i.e. not according to what was recorded), and blurred sound information from sound sources located at center stage.
To date, apparently the most direct attempt to solve the audio image problem in stereophonic systems is disclosed in Sorkin U.S. Pat. No. 3,478,167, entitled "Three Speaker Stereophonic Audio System". That system comprises a conventional stereophonic audio system with an additional third speaker. This third speaker is placed directly in front of the listener and is fed with both left and right channel information, thus insuring that sound recorded from central sources will be played back through a centrally located speaker. The remaining two speakers are placed on either side of the listener and are fed with the respective left and right channel information as well as the inverted sound information of the opposite channel. The inventor claimed that the inverted signal cancelled the oppositely phased identical audio signal emanating from the opposite pair of speakers, to enhance in an undisclosed manner the stereophonic system.
However, this system does not provide A.O.A. and D.I. sound information in proper proportion and thus does not create the true three dimensional realism which the present invention does. The Sorkin system at best wraps the flattened sound stage image around the listener in an 180 degree arc. The Sorkin system creates further sound image confusion because of varying amounts of opposite channel cancellation arising from the sound shadowing of the listener's head and the directionality of the ears.
The present invention solves the soundstage imaging problem directly and with elegant simplicity. Other prior patents of possible interest are cited below:
______________________________________ PRIOR ART PATENTS U.S. Pat. No. Patentee(s) Issue Date ______________________________________ 2,710,662 M. Camras June 14, 1955 2,924,660 Daniel Abrams February 9, 1960 3,066,189 R. H. Ranger November 27, 1962 3,238,304 Mitsura Yaita, et al March 1, 1966 3,478,167 M. Sorkin Nov. 11, 1969 3,710,034 E. J. Murry January 9, 1973 3,725,586 Kazumi Iida April 3, 1973 3,892,624 Satoshi Shimada July 1, 1975 3,892,917 Hiromi Sotome July 1, 1975 3,985,960 Robert Lee Wallace, Jr. October 12, 1976 ______________________________________